1. Field of the Invention
The present invention is generally related to a circuit arrangement for reducing the effective capacitance of a power switch and, more particularly, for reducing the effective capacitance of a power switch connected in series association with an inductor.
2. Description of the Prior Art
As is well known to those skilled in the art, the inherent characteristic of an inductive component, such as a motor winding or a relay, is that an electric current will continue to flow through the inductor after a switch connected in series with the inductor is opened for the purpose of causing current through the inductor to cease. This characteristic of inductive components is typically compensated by providing a snubber circuit (i.e. a means for dissipating the stored energy in the inductor) in parallel with the inductor. If a capacitor is connected in series with the inductor, the capacitive and inductive components combine to create a resonant circuit arrangement that can result in ringing (i.e. resonant oscillation). If the inductor is used in a circuit which is particularly sensitive to high frequencies, this ringing is particularly deleterious. In certain types of circuits, high frequency signals are injected into the circuit to pass through the inductors so that certain parameters of the circuit can be measured. The creation of high frequencies in the circuit as a result of the resonance created by the inductive and capacitive components can be particularly disadvantageous in these types of circuits.
U.S. Pat. application Ser. No. 07/658,482, which was filed on Feb. 20, 1991 by Harris and assigned to the assignee of the present application, describes a means for determining the position of a rotor in a switched reluctance motor by injecting a high frequency signal into the stator windings of the motor. The position of the rotor poles, relative to each stator pole winding, will affect the amplitude and phase of the high frequency injected signal and enable the rotor position to be determined as a function of these effects. In a circuit arrangement of this type, the switch capacitance parallels the coupling capacitor and adds to the coupling capacitance to set the resonant frequency used to determine the rotor position. The inductance of the motor winding pairs reacts with the circuit series capacitance; the intended coupling capacitors and the parasitic switch capacitance. The signal is injected into a circuit which is detuned by the operational variation of the switch capacitance. Further, the signal is divided inversely to the value of each capacitance. The signal passing through the switch capacitance is lost and thus adversely affects the signal level or signal to noise ratio of the detection process. Connecting a diode in series with this switch reduces the effective capacitance value by more than an order of magnitude.
U.S. Pat. No. 5,075,610, which issued to Harris on Dec. 24, 1991, discloses a switched reluctance motor control circuit that possesses the capability of recovering energy that would otherwise be wasted during the operation of the motor. The circuit for the switch reluctance motor is provided with a connection between a second end of each stator winding and a first end of an associated other stator winding. The purpose of this connection is to permit the flow of current from a phase winding to an energy storage device following the disconnection of the phase winding from a primary power source. Because of the inductive characteristic of the phase windings in a switched reluctance motor, the current through the winding does not immediately cease when the winding is disconnected from the power source. Instead, the inductive characteristic resists the immediate cessation of current following the opening of an associated switch. That continued current is directed to an energy storage device, such as a capacitor, for the purpose of raising the voltage at the first, or input, end of another stator winding.
U.S. Pat. No. 4,609,859, which issued to Williams on Sep. 2, 1986, discloses a conduction motor drive circuit which has a pair of windings associated with each phase of the motor, with the windings of each pair being connected in antiphase. The circuit comprises a DC supply, respective semiconductor switches in series with each motor winding for supplying current from the supply to the associated winding in only one direction and frequency control circuitry for switching the switches on and off so as to cause the winding of each winding pair to conduct current alternately and in opposite directions at a frequency which may be varied so as to change the speed of the motor.
In many applications where current flowing through an inductive component is to be switched on and off, the switching function is performed by a solid state switch. As is known to those skilled in the art, power switches have relatively high parasitic capacitances. Although field effect transistors (FET's) and insulated gate bipolar transistors (IGBT's) are usually selected for their relatively low parasitic capacitance characteristics, the use of these semiconductive devices in applications which require high current carrying capability requires that the semiconductive components comprise relatively large silicon areas which, in turn, result in relatively significant parasitic capacitances of several nanofarads. Parasitic capacitance in a semiconductor switch retards its turn off and turn on times when the switch is used in association with resistive, inductive or capacitive loads. When used in association with an inductive load, the parasitic switch capacitance combines with the inductive load to constitute a resonant circuit which must be damped by a snubber network. If the high frequency ringing that results from this combination of components is not damped, it must be tolerated during the operation of the circuit.
It would be significantly beneficial to the design of an electrical circuit arrangement, wherein a semiconductive power switch is connected in series with an inductive component, if a means is provided to mitigate the effect of the parasitic capacitance of the power switch.